Contribution of irreversible non-180° domain to performance for multiphase coexisted potassium sodium niobate ceramics

Despite the dominance of lead-based piezoelectric materials with ultrahigh electric-field-induced strain in actuating applications, seeking eco-friendly substitutes with an equivalent performance remains an urgent demand. Here, a strategy of regulating the irreversible non-180° domain via phase engineering is introduced to optimize the available strain (the difference between the maximum strain and the remnant strain in a unipolar strain curve) in the lead-free potassium–sodium niobate-based piezoelectric ceramics. In situ synchrotron X-ray diffraction and Rayleigh analysis reveal the contribution of the non-180° domain to available strain in the tetragonal–orthorhombic–rhombohedral phase boundary. The reducing orthorhombic phase and increasing rhombohedral/tetragonal phase accompanied by the reduced irreversible non-180° domain are obtained with increasing doping of Sb5+, resulting in an enlarged available strain due to the significantly lowered remnant strain. This optimization is mainly attributed to the reduced irreversible non-180° domain wall motion and the increased lattice distortion, which are beneficial to decrease extrinsic contribution and enhance intrinsic contribution. The mesoscopic structure of miniaturized nanosized domain with facilitated domain switching also contributes to the enhancement of available strain due to the improved random field and decreased energy barrier. The study will shed light on the design of lead-free high-performance piezoelectric ceramics for actuator applications.


Response Letter (NCOMMS-23-52432)
Thank you very much for handing our manuscript and providing valuable comments.
Here, we have revised the article actively and in detail with additional measurements and analysis according to your suggestion, to further improve the manuscript.We hope that these changes will fully address the concerns.All the reversions have been highlighted in red within the manuscript.
Reviewer#1: Potassium sodium niobate (KNN) ceramics have been identified as one of the most promising lead-free piezoelectric candidates owing to their high Curie temperature and large piezoelectric properties.Surprisingly, different performance can be obtained in KNN-based ceramics even with the same phase-coexistence state, illustrating that the origin of high performance has not been sufficiently addressed apart from phase coexistence.This manuscript has thoroughly reported a crucial issue about irreversible non-180 o domains' contribution in KNN-based ceramics.The authors applied experimentally to successfully illustrate the correlation between irreversible non-180 o domains and electrical properties in high-performance KNN ceramics with phase coexistence.It is a meaningful work with systematic characterizations and reasonable discussions.I think that the result is interesting, and explains the main mechanism of the contribution to high performance in KNN ceramics.However, there are still some fatal weaknesses in the current manuscript that need to be clarified before publication: Reply: Thank you very much for carefully reading and valuable comments about this work.According to your recommendations, we positively responded to your questions, as follows.
(1) The major findings in this work should be further discussed.It is interesting to note that ferroelectricity enables polycrystalline materials to exhibit piezoelectricity, but this ferroelectricity limits the level of practically achievable displacement bound by the difference between Spos and Srem for most actuator applications.Spos involves intrinsic piezoelectric strain which mainly comes from crystal lattice distortion and extrinsic domain switching including irreversible and reversible non-180 o domain switching.
The reversible non-180 o domains switch back, and the irreversible non-180 o domains align along the electric field direction, resulting in Srem.As the irreversible non-180 o evolution has a great influence on the strain, the available strain (Spos -Srem) should be emphasized, which is important for most actuator applications.A more sufficient literature review is suggested.
Reply: Thank you for your reminding.Indeed, the available strain for the piezoelectric actuators is the difference between poling and remanent strain (Spol -Srem), which could be greatly affected by both intrinsic lattice distortion and extrinsic domain evolution.
With the modification of irreversible non-180 o domain, the Srem changes as well, exhibiting influence on available strain.Thus, the change tendency of Spol -Srem is added in this work.Significantly, Spol -Srem gradually increases with decreasing contribution of irreversible non-180 o domain from reduced O and increased R/T phases, which can effectively reduce Srem while Spol could maintain because of the R-O-T phase boundary.
For better discussion the important parameter as well as domain evolution, more sufficient literature review and systematical discussion have been conducted, as shown in the revised manuscript and below: "a strategy of regulating the irreversible non-180 o domain via phase engineering is introduced to optimize the available strain (Suni-Srem) in the lead-free potassium-sodiumniobate-based piezoelectric ceramic.""Generally, the actuating performance is assessed merely by the difference between poling strain (Spol) and remanent strain (Srem) (See Fig. 1a), which can be estimated via the following equation: where the Emax is the maximum electric field and d33* is the converse piezoelectric coefficient.Obviously, the higher Spol and the lower Srem, the better actuating performance.Namely, the actuating performance can be modified by enhancing Spol and/or restricting Srem.Spol can originate from multiple mechanisms, including the converse piezoelectric effect, non-180 o domain-wall motion, electrostriction, and possible electric-field-induced phase transition.On the other hand, the Srem is mainly generated by irreversible non-180 o domain evolution which can keep the orientation after withdrawing the electric field.Compared to the complex mechanisms for Spol, Srem with a rather straightforward contribution of the irreversible non-180 • domain might be easier to regulate.Modifying the irreversible non-180 o domain could be an effective approach to optimize available strain.""Significantly, a high available strain is realized with reduced irreversible non-180 o domain and decreased domain size based on increased R/T and decreased O phase." "Improved piezoelectric and ferroelectric properties are achieved for x=0.045.
Meanwhile, strain evolution is further emphasized since it is a figure of merit for piezoelectric actuators.From the unipolar strain curves (Fig. 2g), Srem drops from 0.024% to 0.019% when x increases from 0.04 to 0.05 along with little changed Suni (~0.206%), contributing to enhanced Suni-Srem, where the largest value of 0.187% is observed for x=0.05 (Fig. 2h-2i).The high reversible strain achieved is favorable for the piezoelectric actuating applications."(2) As the R-O-T phase boundary is the basis of the discussion for domain evolution, the identification is not enough only with XRD and εr-T curves.More evidence should be adopted with accurate measurements.It is reported that temperature-dependent XRD and thermally stimulated depolarization current (TSDC) are effective methods for detecting phase structure, which may make the discussion more compelling.1), which is verified by the Rietveld refinement (Fig. 2a, Supplementary Fig. 3, and Table 2)."

Reply
"The R-O-T phase boundary gradually changes to the O-T phase, T phase and cubic phase finally with increasing temperature, leading to the reduced non-180 o domain and weakened domain response (Fig. 2f)."Supplementary Fig. 2 Normalized temperature dependence of depolarization current (jTSDC) of the ceramics.
(3) Another important issue is that this current manuscript is not well-organized including the text and pictures.In particular, some data for assisting with discussion should be moved into Supplementary materials, such as relaxor characterization, et al.
Reply: Thank you for your reminding.All the text and pictures have been reorganized to emphasized the relationship between available strain and irreversible non-180 o domain.The room-temperature XRD, εr-T curves, relaxor characterization, electrical properties, grain morphology haven been moved into Supplementary material.And the focuses including phase structure, available strain, In-situ X-ray diffraction, Rayleigh analysis and domain morphology have been carefully discussed in manuscript.The details are presented in the revised manuscript and supplementary material.
(4) The domain evolution is focused on and discussed in this paper.However, the domain structure characterization is lacking.More evidence, such as domain switching experiments, are recommended to be added.
Reply: Thanks for your reminding.The domain morphology has been added with PFM and TEM.Refined domain is observed with decreasing O and increasing R/T phases.
And the domain switching is promoted yielding from lowered energy barrier, which has been revealed by the lithography process under the bias of 2 V, 4 V, 6 V and 8 V.
The detailed discussion has been presented in the revised manuscript and below: "With optimized available strain from the modified irreversible non-180 o domain, the domain morphology as well as switching behavior under bias is further explored.The virgin domain structure is characterized as presented in Fig. 4a-c.Combining with the amplitude and phase images (Supplementary Fig. 11a-c), a refined domain is observed with increasing Sb 5+ content, due to the reduced O phase and strengthened diffusion instead of grain evolution that changes little (Supplementary Fig. 12).Then, the lithography process is adopted at the regions under the bias of 2-8 V to analyze the domain switching behavior (Fig. 4d).Notably, although domain switching obvious happened at 4 V for x=0.04 and 0.045 while little switching is found at 2 V, better switching is obtained for x=0.045 than that for x=0.04 (Fig. 4e-f and Supplementary Fig. 11d-e).The promoted domain switching is mainly generated by the modified phase content and diffusion behavior with a lowered energy barrier, which is also the origin of high piezoelectric performance (Supplementary Table 3).However, poor domain switching is observed for x=0.05 due to diffused behavior which can make the domain recover easily after withdrawing bias (Fig. 4g and Supplementary Fig. 11f).Meanwhile, optimized piezoelectric response is also gained with the bias voltage-induced piezoresponse hysteresis and phase loops (Fig. 4h, i).
To reveal the domain morphology more accurately, TEM images are supplied as presented in Fig. 4j-l.A nano-sized stripe domain pattern is observed in all the ceramics along with uniformly distributed elements (Supplementary Fig. 13), conforming to the reported system with high performance from diffused R-O-T phase boundary.The nano-sized stripe domain is further verified according to a regular fluctuated intensity at the regions marked by dotted lines (Fig. 4m-o), and the domain size decreases from 27 nm for x=0.04, to 17 nm for x=0.045 and 9 nm for x=0.05 finally (Fig. 4p)." (5) It is insufficient as the authors only present ferroelectricity, small signal d33 and strain with different phase structures and domain evolution.Other important parameters, such as piezoelectric constant, should be also provided, which is closely related to irreversible non-180 o domain switching and phase boundary.
Reply: Thanks for your reminding.The parameters including d33, kp, Qm, εr, and tanδ have been added, which are crucial to piezoelectric materials, as shown the revised manuscript, supplementary material and below: "Especially, the variation of the non-180 o domain could yield different performances (Supplementary Table 3 and Fig. 5).Improved piezoelectric and ferroelectric properties are achieved for x=0.045."(7) Some discussions need to be revised for more rigorous and logical language.

Supplementary
Reply: Thanks for your reminding.The description and discussion of the whole manuscript have been revised with more sufficient literature review and powerful evidence including In-situ X-ray diffraction, Rayleigh analysis and domain morphology as well as switching, to reveal the contribution of irreversible non-180 o domain to available strain in more depth.The detailed modification has been presented in the revised manuscript.
(8) The measurement and characterization are too simple.Detailed information is required.
Reply: Thank you for your reminding.More details have been provided for measurement and characterization, as presented in revised manuscript and below: "Structure characterizations The XRD patterns were gained by X-ray diffraction (XRD) machine with Cu kα radiation in the θ-2θ scan mode (Bruker D8 Advanced XRD, Bruker AXS Inc., Madison, WI).In-situ Synchrotron X-ray diffraction under 0 and 3Ec was performed using Source BL02U2 (photon energy 18 keV) beamline in Shanghai Synchrotron Radiation Facility.
A thermally stimulated depolarization current measurement was conducted using an electrometer/high resistance meter (Keithley 6517B, Keithley Instruments, Inc., Cleveland, OH).Vertical piezoresponse force microscopy (VPFM) was conducted by a commercial microscope (MFP-3D, Asylum Research, Goleta, CA), applied to a conductive Pt-Ir-coated cantilever PPP-NCHPt (Nanosensors, Switzerland).During the litho process, a negative voltage of 20 V was used first, and then a series of positive voltages (2 V, 4 V, 6 V, 8 V) were employed in the oriented region.The local piezoresponse hysteresis loops were studied in the switching spectroscopy piezoresponse force microscopy (SS-PFM) mode, using triangular-wave high voltage.
The transmission electron microscopy (TEM) specimen was prepared by mechanically polishing to around 20 μm in thickness followed by argon-ion beam milling (Gatan PIPS 695, Gatan Inc., USA) with an operating voltage of 0.1-6 kV to reach electron transparency.The TEM was carried out using a high-resolution transmission electron microscope (JEOL 2100F, JEOL, Japan) operated at 200 kV.A field-emission scanning electron microscope (FE-SEM) (JSM-7500, Japan) was applied to characterize the morphology of the surface.
The d33 was measured with a commercial Berlincourt-type d33 meter (ZJ-3A, China) for the poled samples.The ferroelectric hysteresis (P-E) loops, d33-E, and strain curves (S-E) were measured by a ferroelectric tester (aixACC TF Analyzer 2000 E, Germany).
Following the Rayleigh relationship under the sub-switching conditions, bipolar electric fields with amplitudes smaller than 1/2 Ec were applied to the samples.Then increasing the applied electric fields, the unipolar S-E curve under a large electric field with the slope of a linear part can be obtained and estimated." (9) In Table S1, the values of TR-O seem to be not so accurate.The authors should provide more accurate values or adopt more accurate measurements.
Response: Thanks for your reminding.It can be noted that R-O phase transition presents a diffusion behavior with a broad temperature region in KNN-based ceramics [Chem. Soc. Rev., 2020, 49, 671, Nat. Commun., 2021, 12], which is different from the sharp peaks of O-T and T-C phase transition during εr-T curves measurement.Therefore, the TR-O is an approximate value, not an exact one.
(10) There are some expression and grammar problems in the manuscript.The language needs to be polished carefully.
Reply: Thanks for your reminding.The expression and grammar of the whole manuscript have been carefully checked and revised.Then, the manuscript has been also improved by an English native speaker.The details have been presented in the revised manuscript.
Reviewer #2: Recently, the origin of high performance in KNN has been an interesting topic in the community.Generally speaking, the contribution can be broadly divided into the intrinsic and extrinsic contributions.The intrinsic part is related to lattice distortion while domain evolution belongs to the extrinsic contribution.Many recent works revealed the role of domain response in high-performance KNN, such as polar nanoregions (PNRs) (Energy Environ. Sci., 2018, 11, 3531-3539) and "nanoscale strain domains" (Adv.Mater., 2016, 28, 8519-8523).However, few studies focus on the domain type in KNN with high performance, especially the irreversible non-180 o domain.In this manuscript, the authors attempt to disentangle the question of "the effect of irreversible non-180 o domain on performance in lead-free KNN ferroelectrics" and proceed quite successfully on the basis of a phenomenological description centrally involving the domain characteristic of high performance KNN with multiphase boundaries.It is clearly noticed that the basic underlying idea is the easily-neglected problems about the relationship between irreversible non-180 o domain and performance in domain engineering, which is meaningful for high-precision lead-free actuator.The manuscript is well-organized and technically sound, and appropriate in supporting their claims.I would like to recommend its publication in Nature Communications after the authors have addressed the following concerns: Reply: We appreciate the reviewer for the interest and the valuable comments, which will definitely improve this paper.
(1) Since the explanation mainly depends on the domain configuration and its response, more direct evidence of the domain should be provided.TEM and PFM show a remarkable ability to observe and manipulate domain structure, which are widely adopted to detect the domain structure in piezoelectric materials.I suggest authors add the direct evidence of domain by TEM or PFM.
Reply: Thanks for your constructive suggestion.The domain configuration has been studied by PFM and TEM, which provide strong evidence for exploring the relationship between domain and performance in the ceramics.The related revision has been shown in the revised manuscript and below: "With optimized available strain from the modified irreversible non-180 o domain, the domain morphology as well as switching behavior under bias are further explored.The virgin domain structure is characterized as presented in Fig. 4a-c.Combining with the amplitude and phase images (Supplementary Fig. 11a-c), a refined domain is observed with increasing Sb 5+ content, due to the reduced O phase and strengthened diffusion instead of grain evolution that changes little (Supplementary Fig. 12)." "To reveal the domain morphology more accurately, TEM images are supplied as presented in Fig. 4j-l.A nano-sized stripe domain pattern is observed in all the ceramics along with uniformly distributed elements (Supplementary Fig. 13), conforming to the reported system with high performance from diffused R-O-T phase boundary.The

:
Thank you for your reminding.Besides XRD and εr-T curves, the temperaturedependent XRD patterns and jTSDC have been measured to further analyze the phase structure.According to the temperature-dependent XRD patterns, the phase transition from R-O-T, to O-T, T and Cubic phases finally happens with increasing temperature.And, decreasing TO-T and TC are obtained with increasing Sb 5+ from jTSDC, which is almost consistent with the results of εr-T curves.The detailed discussion has been presented in the revised manuscript, Supplementary materials, and below: "Room-temperature phase transitions including TR-O and TO-T are observed for x=0.035-0.055,indicating the R-O-T phase boundary (Supplementary Fig. 1c, Fig.2 and Table

Fig. 4
Fig. 4 PFM amplitude images of the ceramics with a x=0.04, b x=0.045, and c x=0.05.
Figure 4(a)~(c), the legends are missed.Please check it carefully.Reply: Thanks for your reminding.The details are carefully checked, and the missed legends have been added, as shown in the supplementary material: Supplementary Fig. 5 a P-E loop.b Electric field-induced d33 curves.c Bipolar strain curves.d Pr and EC. e d33, f Spos and Sneg for the ceramics.

Fig. 4
Fig. 4 PFM amplitude images of the ceramics with a x=0.04, b x=0.045, and c x=0.05.

Table 3 .
Electric properties for the ceramics.